Viral hemorrhagic fever (VHF) activates the innate immune system triggering an exuberant release of cytokines and other permeability factors that destabilize the endothelial barrier. The loss of vascular integrity results in non-cardiogenic edema, shock, multi-organ failure and death. While the relationship between VHF- induced endothelial breakdown and mortality has been studied in multiple forms of hemorrhagic fever, it has been well established in hantavirus; specifically in New-world hantavirus pulmonary syndrome (HPS) which is characterized by fever and vascular leakage resulting in noncardiogenic pulmonary edema followed in severe cases by shock and death2. In the past year, significant discoveries have been made regarding the mechanisms by which hantaviruses infect endothelial cells and lead to their breakdown. In 2010, Mackow and associates reported that hantaviruses directly enhance the internalization of VE-cadherin in response to VEGF and thereby dissociate endothelial cell adherence junctions which regulate vascular permeability3. Navigen Pharmaceuticals is developing a therapeutic protein, Slit2N, which we have shown to reduce vascular leak resulting from cytokine- induced VE-cadherin internalization4. The recent evidence that hantaviruses cause internalization of VE- cadherin combined with Navigen's discovery that Slit2N can prevent VE-cadherin internalization leads us to believe that we must make testing Slit2N in hantavirus a priority. This conclusion is supported further by the fact that we have shown that Slit2N can reduce mortality in models of conditions in which cytokine storm is believed to induce massive endothelial breakdown and vascular leak4. Navigen believes that Slit2N may have broad-spectrum efficacy in the treatment of viral hemorrhagic fevers as there is evidence that VE-cadherin internalization plays a role in the vascular leak associated with other hemorrhagic fevers including Dengue5 and hemorrhage-inducing agent anthrax6. Additionally, Slit2N has been shown to prevent mortality from vascular leak in a number of other conditions associated with massive cytokine storm, including sepsis4. We propose to use Hantavirus pulmonary syndrome to illustrate efficacy in a form of VHF with the clearest link to Slit2N's mechanism of action, preventing the vascular endothelial cell junction breakdown caused by hantavirus-induced VE-cadherin internalization. Identification of the most efficacious dose and dose schedule for Slit2N in treating HPS could lead to the first approved therapy to treat Hantavirus Pulmonary Syndrome and would set the stage for expanding testing to other forms of VHF in which vascular leak is believed to contribute significantly to mortality (e.g. Ebola virus, Lassa fever virus, Dengue virus, etc.). In this project, we would identify the most efficacious dose of Slit2N in treating HPS in Syrian hamsters. Once the optimal dose is identified, we will experiment with dose timing to determine how late in the course of the disease Slit2N can be effective. We will also test Slit2N in conjunction with the anti-viral ribavirin in an effort to determine whether, as we hypothesize, Slit2N in combination with an anti-viral will result in better outcomes than with either therapy alone. With the exception of basic pharmacokinetic studies to identify appropriate dose ranges in hamsters, IND-enabling pharmacokinetic and toxicology work will be supported under a separate project. Specific Aim 1: Investigate Slit2N pharmacokinetics (PK) and 7-day tolerability in hamsters. Specific Aim 2: Investigate dose-response efficacy of Slit2N against ANDV. Specific Aim 3: Investigate efficacy of delayed therapy with Slit2N against ANDV. Specific Aim 4: Investigate efficacy of Slit2N in combination with ribavirin.